This invention relates to weighing devices, or checkweighers, for conveyor lines and more particularly to an arrangement in which a segment of such a line is supported atop a weight sensor.
Weighing devices in an assembly line are situated so as to sample weigh, or weigh individual commodities passing over the line for controlling, automatic filling and loading devices. They are of various types and kinds but perhaps one of the most widely used is the integrated belt segment and weighing device. The latter can be inserted at any point in a line and it provides a separate belt drive to continue the movement of commodities over the weighing head and to adjacent belt line systems.
These portable belt-weighers, however, are known to have a high degree of error in their weight scale readings. Attempts to reduce the errors by scale compensation circuits has proven unsuccessful, largely because such errors are caused by structural deficiencies, by the off-set loading of the commodities themselves and are inherent to the system design.
In the majority of prior art belt-weighers, a weighing platform and its sensor are both placed centrally under a section of the horizontal portion of a moving belt. The belt is designed with sufficient slack between supporting rollers to allow the commodity on the belt to rest momentarily on the weighing platform for the sensor to take a weight reading. In practice, to obtain the necessary slack without severe motor slip the horizontal length of the belt assembly is required to be at least four feet between supporting end rollers. This requirement gives rise to two error producing conditions. One, the off-set loading (moment effect of a commodity resting on the end of a belt assembly) of such a long conveyor belt produced erroneous weigh readings which are unpredictable when measuring loose material (e.g., sugar, sand, etc.). Two, the belt slack required to permit the commodity to contact the weight head is variable and dependent upon the number of articles simultaneously present on the belt during weighing. Also imperfections in the belt itself tended to introduce errors in weight readings. Attempts to solve the foregoing problems by increasing the spacing between articles required also that the belt speed be increased in order to maintain the same production output. This solution was both impractical and tended to worsen the reading errors.
Thus, it is an object of the present invention to provide a belt-weigher suitable for insertion in a conveyor line which overcomes the foregoing problems and reduces the potential for weighing errors due to mechanical problems and other inherent problems of the assembly design.
It is also another object of this invention to provide a checkweigher which is relatively small by comparison to prior art conveyors and exhibits a high degree of accuracy in weight reading.